Anomalies in the overall shape of the eye can cause visual disorders. Hyperopia ("farsightedness") occurs when the front-to-back distance in the eyeball is too short. In such a case, parallel rays originating greater than 20 feet from the eye focus behind the retina. In contrast, when the front-to-back distance of eyeball is too long, myopia ("nearsightedness") occurs and the focus of parallel rays entering the eye occurs in front of the retina. Astigmatism is a condition which occurs when the parallel rays of light do not focus to a single point within the eye, but rather have a variable focus due to the fact that the cornea is aspherical and refracts light in a different meridian at different distances. Some degree of astigmatism is normal, but where it is pronounced, the astigmatism must be corrected.
Hyperopia, myopia, and astigmatism are usually corrected by glasses or contact lenses. Surgical methods for the correction of such disorders are known. Such methods include radial keratotomy (see, e.g., U.S. Pat. Nos. 4,815,463 and 4,688,570) and laser corneal ablation (see, e.g., U.S. Pat. No. 4,941,093).
Another method for correcting those disorders is through the implantation of polymeric rings in the eye's corneal stroma to change the curvature of the cornea. Previous work involving the implantation of polymethylmethacrylate (PMMA) rings, allograft corneal tissue, and hydrogels is well documented. One of the ring devices involves a split ring design which is inserted into a channel previously dissected in the stromal layer of the cornea. A minimally invasive incision is used both for producing the channel and for implanting the implant.
The use of intrastromal corneal rings made completely of certain hard, hydrophobic polymers is known. For instance, in the article "Intrastromal Implantation Eines Justierbaren Kunststofforings Zur Homhautrefraktionsanderung", Hartmann et al., Kongress der Deutschen Gesellschaft fur Intraokularingen Implantation, delivered by H. Fryler et al., Springer-Verlag, Wien, pages 465-475, the use of intrastromal ring implants of rings of silicone, polymethylmethacrylate ("PMMA"), and fluorocarbons (TEFLON.RTM., a PTFE available from DuPont). Other disclosures of the use of PMMA in such intrastromal rings is found in U.S. Pat. Nos. 4,452,235 to Reynolds; 4,671,276 to Reynolds; 4,766,895 to Reynolds; and 4,961,744 to Kilmer et al. These documents do not suggest the use of multiple layers of differing materials in the intrastromal corneal rings.
The use of soft polymers as intrastromal inserts is not widely known. For instance, U.S. Pat. No. 5,090,955 to Simon, suggests an intrastromal corneal ring which is made by introducing a settable polymer into an intrastromal channel which has been previously made and allowing the polymer to set. This procedure does not allow the surgeon to specify the size of the resulting ring nor is it a process which allows control of the flowing polymer within the eye.
Temirov et al, "Refractive circular tunnel keroplasty in the correction of high myopia", Vestnik Oftalmologii 1991: 3-21-31, suggests the use of collagen thread as intrastromal corneal ring material.
They specifically do not suggest the use of soft or hydrophilic polymers insertable into the cornea as intrastromal corneal rings.